Launch VS Code, use the Install from VSIX command in the Extensions view command drop-down, or the Extensions: Install from VSIX... command in the Command Palette, and point to the .vsix file (i.e. plantuml-2.17.5_vsixhub.com.vsix).

To make things easier, you could also make an alias to refer to plantuml without typing the full java syntax. Do this by adding the following line to your profile (either .bashrc or .zshrc, based on which shell you use):

Plantuml Vscode Download